How do I get past the Tret Tree?

Not sure what you are having troubles with exactly, but just remember there are vines you can climb on the branches. If that doesn't help try searching "tret tree" on youtube and watch some of those videos.

This place can be a little confusing, as the exterior boughs are blocked by foliage and this obscures the path. First things first, climb to 2F and exit out the right side. There'll be a vine connecting to the 3F branches here, which can be taken on into the interior again. The chest in the center has a Healing Ring, but the leaf stepping stone Isaac takes breaks after one try, and he'll fall down to 2F again -- it's worth the backtracking! Climb up to 4F when ready. The two paths here both lead to 5F, but the right path leads to a Nut and Jupiter djinni (Breeze) who must be fought to be tamed; the left path is the real way to go though. Go across the middle leaf until it's cracked, then jump back onto it to fall all the way to B1 where the boss is. ["Retreat" psynergy can be used if you're not sure about winning here.]

When you finish fighting Tret at the bottom of the Tree, he will tell you to leave before the forest dies. You will make a trip up north to Imil and complete the mission at the Mercury light house. after the becon is lit, return to Imil and look in the first house in the town where you first meet Mia and there will be an empty bottle. Go to the light house and fill the bottle up and return to Tret. Use the water on him and he will heal and return the people to normal. I hope this helps. ~Midnight Wind~

If you have problems beating Tret you can also go to Imil and get Mia first. Though I recommend first letting Mia join your party, then travel back to fight Tret, and only after that return to Imil to finish the lighthouse. It is possible to do the lighthouse first, but it might be to difficult at that point.

Throughout the level there are spider webs with platforms for you to jump on, and if you jump on a platform that's broken, you'll fall to the level below. Keep that in mind as you go through Tret. Also, on the top floor, there will be a big spiderweb. Remember the hole in the middle of each level of the tree? Your objective is to fall through that hole, eventually leading to a new level. So when you're on the top level, plan your jumps so that you'll fall through the spider web right in the middle.

Hope this helps!

I'm not sure why the other answers have yet to help you, but outside all that information,
if you're already done with tret but the game doesn't seem to continue, get an empty bottle from the 1st house you see when you enter Imil, light the mercury lighthouse there (meaning complete it), and water will flow in the spring at the lighthouse's base, use the empty bottle there to get Herme's water, use it on tret.

You should use retreat Psyenergy

If you haven't been to and completed Mercury lighthouse, you can't finish the area, you can get to Tret w/o completing mercury though.